Plastics are finding increasing use in manufactured goods. For example certain automobiles have plastic body panels, aircraft have plastic interior paneling and even exterior skin panels may be made of plastic composites. While plastics offer several excellent properties such as light weight, formability, and low cost, plastics also have significant short comings. In general, plastic surfaces are not as hard or abrasion resistant as steel surfaces, for example. Further, while some plastics may be transparent, glass which is much heavier and more expensive remains the material of choice in certain critical applications such as safety glass in automobiles and in passenger aircraft windshields.
The primary cockpit windshields in commercial airplanes are typically made of multi-pane laminated glass. Glass is used because of its strength, scratch and wiper-blade abrasion resistance, and chemical and environmental resistance. These properties insure that good vision is maintained through critical cockpit windshields, especially where wipers are used for rain removal. Glass has limitations, however, in terms of formability and is not always the lightest and/or least expensive material for windshield construction. Cockpit windshield shapes have remained virtually the same during the last several decades because of limitations in glass formability. Switching to polymeric materials, such as stretched acrylic or polycarbonate, could lead to lighter, less expensive windshields and permit greater flexibility in terms of windshield shape. Reshaping of cockpit windshields would lead to improved airplane cab aerodynamics with associated aero-drag reduction and hence improved airplane fuel efficiency. Improved cockpit aerodynamics would also lead to reduced exterior noise generation and hence might lower internal cabin noise levels. Reduced noise levels may make airplane travel more comfortable to the flight crew and passengers.
Aircraft passenger windows, on the other hand, unlike passenger aircraft windshields, are typically made of stretched acrylic (i.e. acrylic substrate) due to its light weight, flexibility and formability. However, acrylic is susceptible to particle (e.g. sand) and water induced erosion and crazing. Moreover, during flight, aircraft windows are subjected to differential pressures caused by the difference in pressure between the inside and the outside of the aircraft. This differential pressure causes the windows to flex and the flexing may cause the windows or any coatings on the windows to crack over a period of time. To avoid the fine cracks allowing the potential for chemicals to attack the acrylic substrate and/or allowing the coating to delaminate from the acrylic substrate, the windows are replaced in routine maintenance. This poses an additional expense and reduces the potential in-service time of the aircraft.
It might be expected that if polymeric laminates were used as windshields, then water absorption into such a windshield and corrosive effects of chemicals could lead to crazing when stress is applied to the windshield, as may be encountered in flight. Crazing or other mechanical damage, such as scratches, can have a deleterious effect on operator vision through the windshield. In addition, scratched windshield would have to be replaced routinely thereby imposing additional repair costs and reducing aircraft in-service time.
Polysiloxane coatings have been used to protect polymeric substrate surfaces from chemical attack, abrasion and wear. Polysiloxane hard coatings, applied using organosiloxane compounds, protect polymeric substrates such as polycarbonate or acrylic from damage caused by abrasion and/or environmental exposure. These solvent-based coatings, typically a few microns thick, are applied through a dip or flow coat or spray process and then dried through a low temperature (150° F.=65.5° C.) bake. As passenger aircraft windshields are stressed under airplane pressurization these polysiloxane coatings are tailored to provide good elongation properties, however, this limits their abrasion resistance. While currently available coatings, when applied to passenger aircraft windshields, exhibit good optical characteristics, their scratch resistance and durability are limited based on field results. They are either prone to cracking or they provide minimal abrasion protection. Both cracking and/or abrasion can lead to de-bonding of the polysiloxane coating due to environmental exposure and result in scratches and crazing of the base acrylic windshield substrate.
Accordingly, it is desirable to develop barrier coatings to protect polymeric substrates like passenger aircraft flight deck windshields against abrasion, chemical attack, and crazing. In addition, in the case of passenger aircraft windshields, the coatings should be reliable and have some means to warn the flight or maintenance crews when they are no longer effective. Further, in aircraft windshields, the coatings should exhibit good adhesion to the polymeric substrate, excellent wear resistance, minimal ultraviolet light-induced degradation, good elongation/flexibility, and resistance to crazing when exposed to sulfuric acid and a host of chemicals used in aircraft cleaning and maintenance. In addition it should have an indicator that shows the coating condition. Furthermore, other desirable features and characteristics of the barrier coatings will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.